Hops, one of the major raw materials for beer, are used to give a characteristic aroma and a refreshing bitter taste to beer. When hops are boiled along with the wort prior to the fermentation process, essential oils which provide the aroma and bitter taste components which provide the refreshing bitter taste are extracted, and subsequently thermal isomerization takes place, whereby the wort comes to have the characteristic hop flavour. In the method described above, the bitter taste components, e.g., .alpha.-acids (Structural Formula 1), a major component of the soft resins of hops, isomerizes to water-soluble bitter taste components, mainly iso-.alpha.-acids (Structural Formula 3), during boiling the wort to give a refreshing sharp bitter taste to beer. As for .beta.-acids (Structural Formula 2), another major component of the soft resins of hops, only a very small amount is extracted and transferred into the wort because its water-solubility is extremely low. ##STR1##
However, hops must be used after storage in the dried form after harvesting and in the form of whole hops, hop powder, hop pellets and so on, since it is difficult to harvest hops all through the year. Moreover, hops are very susceptible to oxidation; when .alpha.-acids or .beta.-acids, major components of the soft resins of hops, are oxidized, the resulting deteriorated resin products such as humulinic acids (Structural Formula 4), tricyclodehydroisohumulones (Structural Formula 5) and hulupones (Structural Formula 6) are extracted and transfer into the wort, and most of them transfer into beer because they are highly soluble in water. ##STR2##
When these deteriorated resin products exist in beer, they deteriorate the quality of the beer. The degree of production of these deteriorated resin products depends on conditions of the hop drying stage, processing stage, storage and other factors. With respect to the production mechanism and the effects on the quality of beer, the present inventors have already investigated in detail (M. Ono, Y. Kakudo, R. Yamamoto, K. Nagami and J. Kumada, J. Amer. Soc. Brew. Chem., vol. 45, pp. 61-69, 1987).
As stated above, hops are susceptible to oxidation, and oxidized hops deteriorate the quality of beer; therefore, degassing and strict temperature control are necessary during the drying and processing stages. For storage, a warehouse equipped with large refrigerating equipment is necessary, which also requires strict control.
On the other hand, there is a method in which only desired components are extracted from hops with organic solvents and used as the hop extracts in place of hops themselves. In this case, the desired components are stored in the form of concentrated hop extracts; therefore, the hop extracts offer the advantages of easy handling and storage space reduction. However, the hop extracts using organic solvents have various problems as follows.
1) Hard resin, tannin, fats, wax and pigments such as chlorophyll are extracted along with the desired components for beer flavour and the hop extracts have poor colour. Beer prepared using the extracts may lack refreshment and excess miscellaneous tastes.
2) It is feared that harmful organic solvents may remain in the hop extracts.
3) Upon distilling off the organic solvent, the flavour components escape, and deteriorated resin products increase due to heating.
On the other hand, as a method of efficient extraction of desired components from natural products without the problem of residual organic solvent, supercritical fluid extraction using carbon dioxide (West German Patent Publication No. 2127618) is known. Examples of published application of this method applied to hops include Japanese Patent Examined Publication Nos. 44864/1973 and 41375/1989 and U.S. Pat. Nos. 4,104,409 and 4,344,978.
However, none of these attempts based on the prior art method aims at producing hop extracts containing the reduced amounts of deteriorated resin products; there is no disclosure other than a supercritical fluid extraction method using carbon dioxide and a production method for hop extracts having a low tannin content.
Based on the method described above, the present inventors attempted to use carbon dioxide in a subcritical or supercritical condition to produce hop extracts which contain the reduced amounts of deteriorated resin products.
It was found, however, that if carbon dioxide is recycled and reused after extraction and separation of hop extracts, the desired object cannot be accomplished because of simultaneous extraction of a part of the deteriorated resin products. On the other hand, if carbon dioxide is not recycled but vented into the atmosphere after separation of hop extracts, the amount of simultaneously extracted deteriorated resin products can be reduced, but a huge amount of fresh carbon dioxide is necessary, which is too expensive for the production of hop extracts. In addition, ventilation carbon dioxide into the atmosphere involves a critical problem of escape of the desired flavour components characteristic to hops.
Although some other methods are available in which the desired components, e.g., soft resins and essential oils, are selectively separated under finely set separating conditions or in which the deteriorated resin products are selectively separated and removed, these methods are undesirable from the economic and operational viewpoints because the equipment is complex and process control is troublesome.
As stated above, by the prior art method, it is difficult to economically and efficiently produce hop extracts containing large amounts of desired components and reduced amounts of deteriorated resin products without a loss of flavour components.